Add support for free-threaded Python (PEP 703)

[Gaganraj2002]

Summary

Build and ship free-threaded Python (cp313t/cp314t) wheels, enabling deptry to work on Python built without the GIL.

Changes:

Release workflow (.github/workflows/release.yml):

• Add free-threaded wheel build steps to linux, windows, and macos jobs
• On Linux, manylinux containers already bundle python3.13t/python3.14t
• On Windows/macOS, install free-threaded Python after the abi3 build to avoid interpreter detection conflicts (maturin-action#313)
• Free-threaded steps are skipped for PyPy matrix entries

Test workflow (.github/workflows/main.yml):

• Add 3.13t and 3.14t to the test matrix
• Skip mypy typing group for free-threaded builds (not yet compatible)

Tests:

• Add tests/unit/test_thread_safety.py with concurrent stress test exercising the Rust import extraction from 8 threads simultaneously

Why no Rust code changes are needed:

The extension already uses PyO3 0.28.2, which defaults to gil_used = false. The Rust codebase has zero global mutable state, zero unsafe blocks, and zero static variables — all functions are pure computations. PyO3 automatically ignores the abi3-py310 feature flag when it detects Py_GIL_DISABLED, producing version-specific cp313t/cp314t wheels instead of abi3 wheels.

Test plan

• Thread-safety test passes locally (8 threads x 50 iterations)
• Existing tests unaffected
• CI: Tests pass on 3.13t and 3.14t with free-threaded Python
• CI: Free-threaded wheels build on Linux, macOS, and Windows